In recent years, a technology for forming a thin film transistor (TFT) using a semiconductor thin film (a thickness of around several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors are broadly applied to electronic devices such as an IC or an electro-optic device, and are particularly developed as switching elements for image display devices at a rapid rate.
A semiconductor thin film that can be used for higher speed operation has been required since a image display device or an image sensor is made larger and higher density of pixels (high definition) is advanced recently. In addition, in order to achieve light-weight or cost reduction, a thin film transistor is applied also to a driver element in the periphery of a display region, as well as a switching element of an image display device.
A method for enhancing electric characteristics such as mobility by forming a semiconductor thin film having a crystal structure, for example, a solid phase epitaxy method or a laser annealing method has been studied.
The solid phase epitaxy method is a method by which an amorphous silicon thin film is formed over a substrate and is heated to become a polycrystal thin film. The heating is conducted at a temperature of about 600 to 1000° C. for a long time, and an expensive quartz substrate that can endure high temperature is required.
Glass substrates are of great promise in terms of costs as compared with quartz substrates or single crystal semiconductor substrates. However, glass substrates have poor heat resistance and are easy to be deformed by heat. Thus, in the case of forming a TFT using a polycrystal semiconductor film over a glass substrate, a laser annealing method is suitable for crystallization of a semiconductor film to prevent a thermal deformation of the glass substrate.
The laser annealing has advantages that the processing time can be shortened to a large degree as compared with another annealing method using radiation heat or conduction heat, and that a semiconductor substrate or a semiconductor film can be heated selectively, so that the substrate is hardly damaged thermally.
Laser oscillators are classified broadly into two kinds, which are a pulsed laser oscillator and a continuous wave laser oscillator, in a standpoint of the oscillation method. Laser light (also, referred to as a laser beam) oscillated from a pulsed excimer laser is used for laser annealing in many cases. The excimer laser has advantages that it has a high output power and repetition irradiation is possible at a high frequency.
Laser light oscillated from an excimer laser has an advantage such as high absorption efficiency to a silicon thin film that is used as a semiconductor film in many cases.
For example, in laser irradiation, laser light is shaped into linear laser light on the irradiation surface by an optical system (such as a beam homogenizer) and irradiation position of laser light is moved relatively to the irradiation surface. This method is superior industrially because an amorphous silicon film having a large area can be crystallized at one time and thus, high productivity is obtained by this method. Hereinafter, laser light having a linear shape on an irradiation surface is referred to as a linear beam.